Integrated optical devices can include silicon waveguides formed on the upper surface of a silicon wafer. FIG. 1 shows such a construction, in which a rib waveguide 10 is formed on a layer of silicon 12. The silicon layer 12 is silicon-on-insulator, having been grown epitaxially over a silica layer 14 within a silicon wafer 16. The entire waveguide is coated for protective purposes with the silica layer 18. As a result, light propagates within the waveguide 10.
The actual distribution of optical energy is in fact within the zone 20. This extends within the upstanding waveguide rib 10, but is principally within the SOI layer 12 and does in fact extend slightly either side of the waveguide rib 10.
Some stray light will inevitably be lost from the waveguide. This will normally propagate within the SOI layer 12, being retained therein by internal reflection. Eventually it may be reflected into a receiver photodiode present on the chip, thus increasing the cross talk signal and decreasing the signal to noise ratio for the device as a whole. The performance of the device could therefore be improved by eliminating such stray light.
It is known to provide locally doped areas within the SOI layer. These act as absorbent areas for stray light, which is then dissipated as heat.